Dual beam laser device for linear and planar alignment

ABSTRACT

A high precision alignment apparatus is provided that utilizes a semiconductor laser device, such as a laser diode, to provide a source laser beam. The alignment apparatus permits the division of a pair of beam components from the source laser beam useful for either linear or planar alignment. A centroid measurement between the beam components provides a corrected reference point that accounts for the inherent instability of the source laser beam to yield a high level of alignment accuracy. For linear alignment purposes, the beam components are collinearly directed. In the alternative, for planar alignment purposes, the beam components may be either collinear or directed in opposite directions, and rotated to sweep respective planar regions. The laser alignment apparatus is capable of providing a level of accuracy heretofore achievable only with gas lasers, while maintaining the economical attributes of commercial semiconductor laser diodes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to laser systems useful for precisionlinear and planar alignment, and more particularly, to an apparatus thatperforms a light energy centroid measurement between a pair of referencelaser beams in order to significantly improve alignment accuracy.

2. Description of Related Art

Laser emitters and detectors are commonly used in industry to performprecision alignment and measurement. In one type of such system, a laserdevice emits a laser beam that can provide a relatively accuratereference line. A detector operated in conjunction with the laser canmeasure displacement between the beam and an object requiring alignmentto the laser axis. Alternatively, a laser device may be oriented with arotating pentaprism or penta-mirror assembly that deflects the laserbeam through a precise 90° angle and sweeps the beam to provide a flatplane that is perpendicular to the input laser beam. The plane can beused as a reference to which the alignment and/or position of otherobjects can be compared. Such laser alignment systems have variousapplications within construction, surveying or manufacturing. In onepossible application, a laser beam alignment device could be used incommercial construction to define a plumb line or a planar laser beamalignment device used to build a wall. Examples of such laser beamalignment devices are disclosed in U.S. Pat. No. 4,676,598 to Markley etal. for MULTIPLE REFERENCE LASER BEAM APPARATUS, and in U.S. Pat. No.4,662,707 to Teach et al. for LIGHTHOUSE STRUCTURE AND COMPENSATING LENSIN REFERENCE LASER BEAM PROJECTING APPARATUS.

Traditionally, the laser beam in such alignment systems is generated bya laser tube containing a suitable active gaseous element, such ashelium-neon gas. The gas is excited by an optical or electrical sourceto emit a low-powered collimated output beam in the red band of thelight spectrum. These so-called gas lasers produce a thermally stablebeam that is useful for producing highly accurate measurements overrelatively long distances (e.g., hundredths of an inch over one hundredfeet of distance). Despite the inherent stability of reference beamsproduced by gas lasers, however, these laser devices are also veryenergy inefficient, expensive, relatively large and fragile.

In view of the noted deficiencies of gas lasers, laser diodes haveincreased in commercial popularity as a suitable alternative. A laserdiode is a semiconductor device, usually of the gallium-arsenide type,that emits coherent light when a voltage is applied to its terminals.Laser diodes are substantially less expensive than gas lasers, and canbe manufactured in a smaller, rugged, more compact package. Asignificant drawback of laser diodes is that they produce a far lessstable beam than gas lasers. In particular, the laser beam does not haveuniform intensity over its cross-section and the energy of the beamdecreases with distance from the center of the beam. Moreover, the rateof change of the intensity is not uniform, and the center of beam energywill periodically shift in accordance with temperature. As a result ofthese deficiencies, a linear or planar alignment device using a laserdiode cannot be expected to achieve the same degree of accuracy as gaslasers.

Prior art devices have sought to overcome the inadequacy of laser diodesin order to approximate the stability of gas lasers. In particular, U.S.Pat. No. 5,307,368 to Hamar for LASER APPARATUS FOR SIMULTANEOUSLYGENERATING MUTUALLY PERPENDICULAR PLANES discloses a laser alignmentdevice having a hollow spindle with an aperture that is rotatablymounted within bearings. Laser light from a laser diode floods theaperture so that only the centermost portion of the beam passes entirelythrough the spindle. The laser alignment device disclosed by Hamarpurports to provide a centered beam of uniform density and circularcross-section. In practice, however, the resulting beam still cannotachieve the stability of gas laser beams due to thermal shifts over timewithin the center of beam energy that passes through the spindle.

Notwithstanding this significant drawback, laser diodes are stillacceptable for most alignment applications. In general, commercialconstruction applications do not require a level of accuracy high enoughto justify the additional expense of a gas laser, and, in theseapplications, a laser beam generated by a laser diode can provide asufficient level of stability. As a result, demand for laser diodes hastotally outstripped demand for gas lasers and, currently, there are fewsuppliers willing to produce gas lasers to satisfy the particularapplications that require greater accuracy than that achievable withlaser diodes.

Accordingly, a critical need exists for a laser alignment apparatuscapable of providing a highly stable and accurate beam suitable for suchcritical linear and planar alignment applications. Such a laseralignment apparatus should be capable of providing a level of accuracyheretofore achievable only with gas lasers, while maintaining theeconomical attributes of commercial laser diodes.

SUMMARY OF THE INVENTION

A high precision alignment apparatus is provided that utilizes acollimated light source, such as a laser diode, to provide a sourcelaser beam. The alignment apparatus permits the division of a pair ofbeam components from the source laser beam useful for either linear orplanar alignment. A centroid measurement between the beam componentsprovides a corrected linear or planar reference point that accounts forthe inherent instability of the source laser beam and yields a highlevel of alignment accuracy.

In a first embodiment of the invention, a planar alignment apparatus isprovided. The apparatus comprises a laser diode providing a source laserbeam and a wedge having first and second faces that intersect at a 90°angle. The beam is directed at the intersection of the first and secondfaces of the wedge, which have surfaces that respectively reflect thefirst and second beam components in opposite directions. The wedge isrotated about a spin axis orthogonal to the reflected beam componentsand approximately coincident with the source laser beam to cause thefirst and second beam components to sweep respective planar regions. Astable planar measurement is obtained by detecting a common centroid ofthe first and second beam components, using a photodetector oriented topermit the first and second beam components to impinge thereon. As thesource laser beam deviates due to thermal or angular effects, the firstcomponent will shift upward while the second component shifts downward,or vice versa. The centroid measurement of the first and second beamcomponents thus represents a weighted average position of the two beamcomponents that can be used to define a stable planar measurement pointfor the planar alignment apparatus.

In a second embodiment of the invention, a linear or axial alignmentapparatus is provided. A source laser beam from a semiconductor deviceis divided into first and second beam components that may be partiallyoverlapping. A first optical element disposed at a predetermined anglewith respect to an optical axis reflects the first beam componenttherefrom in a direction normal to the axis and transmits the secondbeam component therethrough. The optical axis is approximatelycoincident with the source laser beam. A second optical elementcollinearly reflects the reflected first beam component back to thefirst optical element, the first beam component being therebytransmitted through the first optical element. A third optical elementdisposed normal to the axis coaxially reflects the second beam componentback to the first optical element. The reflected second beam componentthereby reflects off of the first optical element in a partiallyoverlapping manner with the first beam component.

A linear or planar alignment measurement is made by detecting a commoncentroid of the first and second beam components. A photodetector isoriented to permit the first and second beam components to impingethereon and provide the common centroid measurement therefrom. Thereby,the collinear and overlapping beams provide a reference for a linearalignment system. Alternatively, by rotating the overlapping beamcomponents about the optical axis, the beam components sweep a planarregion to provide a planar alignment system.

In a third embodiment of the invention, a planar alignment apparatus isprovided. A semiconductor laser source provides a laser beam that isdivided into a first and second beam component directed in oppositedirections. A first optical element disposed at a predetermined anglewith respect to an optical axis approximately coincident with the laserbeam reflects the first beam component therefrom in a direction normalto the optical axis and transmits the second beam componenttherethrough. A second optical element collinearly reflects thetransmitted second beam component back to the first optical element,with the first beam component being thereby reflected therefrom in adirection normal to the optical axis and opposite to the direction ofthe first beam component. The first and second beam components arerotated about the optical axis to sweep respective planar regions, and acommon centroid measurement of the first and second beam components ismade to provide a planar alignment system.

In a fourth embodiment of the invention, a planar alignment apparatus isprovided. A semiconductor laser emitting source provides a laser beamand the laser emitting source is rotated about a spin axis approximatelycoincident with the laser beam at a first rotational rate. The laserbeam is reflected in a direction normal to the spin axis by an opticalelement having a plurality of reflecting surfaces, which is rotatedabout the spin axis at a second rotational rate to cause the reflectedlaser beam to sweep through a substantially planar region. The firstrotational rate is half that of the second rotational rate. A centroidmeasurement of the reflected laser beam provides planar alignmentinformation.

In a fifth embodiment of the invention, a planar or linear alignmentapparatus is provided. A semiconductor laser source provides a laserbeam that is divided into collinear first and second beam polarizationcomponents. A first optical element disposed at a predetermined anglewith respect to the laser beam reflects the laser beam therefrom. Asecond optical element reflects the first beam polarization componenttherefrom back to the first optical element, the first beam polarizationcomponent being thereby transmitted through the first optical element ina reference direction. The second optical element further transmits thesecond beam polarization component therethrough. A third optical elementcoaxially reflects the second beam polarization component back to thefirst optical element, the reflected second beam polarization componentbeing thereby transmitted through the first optical element in acollinear manner with the first beam polarization component in thereference direction. A common centroid of the first and second beampolarization components provides linear alignment information.Alternatively, the first and second beam polarization components may berotated about an axis orthogonal to the reference direction to sweeprespective planar regions. A common centroid measurement of the firstand second beam polarization components is made to provide planaralignment information.

In a sixth embodiment of the invention, a linear or planar alignmentapparatus is provided. A collimated light source provides a laser beamthat is divided by an optical element into collinear first and secondbeam components. The optical element comprises a plurality of surfacesand a partially reflective boundary. The laser beam impinges upon afirst surface thereof and is internally deflected into the opticalelement. A first beam component is reflected from the partiallyreflective boundary and passes through a second surface of the opticalelement in a reference direction. A second beam component is therebytransmitted through the partially reflective boundary and passes througha third surface of the optical element in the reference direction. Acommon centroid of the first and second beam components provides linearor planar alignment information.

In a seventh embodiment of the invention, a planar alignment apparatusis provided. A collimated light source provides a first and a secondlaser beam that are directed in opposite directions. The collimatedlight source is rotated about an axis to cause the first and secondlaser beams to sweep respective planar regions. A common centroid of thefirst and second laser beams provides planar alignment information. Thecollimated light source further comprises a gas laser.

A more complete understanding of the dual beam laser device for linearand planar alignment will be afforded to those skilled in the art, aswell as a realization of additional advantages and objects thereof, by aconsideration of the following detailed description of the preferredembodiment. Reference will be made to the appended sheets of drawingswhich will first be described briefly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a first embodiment of the laseralignment apparatus of the present invention;

FIG. 2 is a front view of a centroid measurement device of the firstembodiment having a first and second beam component impinging thereon;

FIG. 3 a block diagram of a second embodiment of the laser alignmentapparatus useful for planar or linear alignment;

FIG. 4 is a front view of a centroid measurement device of the secondembodiment having a first and second beam component impinging thereon;

FIG. 5 is a block diagram of a third embodiment of the laser alignmentapparatus useful for planar alignment;

FIGS. 6a and 6b are front views of centroid measurement devices of thethird embodiment having a first and second beam component impingingthereon, respectively;

FIG. 7 is a block diagram of a fourth embodiment of the laser alignmentapparatus;

FIG. 8 is a block diagram of a fifth embodiment of the laser alignmentapparatus;

FIG. 9 is a block diagram of a sixth embodiment of the laser alignmentapparatus; and

FIG. 10 is a block diagram of a seventh embodiment of the laseralignment apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention satisfies the critical need for a laser alignmentapparatus capable of providing a highly stable and accurate beamsuitable for critical linear and planar alignment applications. Thelaser alignment apparatus provides a level of accuracy heretoforeachievable only with gas lasers, while maintaining the economicalattributes of commercial laser diodes. In the detailed description thatfollows, like reference numerals are used to describe like elementsillustrated in one or more of the figures.

Referring first to FIG. 1, a first embodiment of a planar laseralignment apparatus is illustrated. The laser alignment apparatuscomprises a collimated light source 12 and a rotating wedge 22. Thelaser device 12 may be a conventional semiconductor device, such as alaser diode, which provides a laser beam A. The wedge 22 has a centralaxis Z aligned approximately coincident with the laser device 12. Sincethe laser beam A provided by the laser device 12 is inherently unstable,it should be apparent that the central axis Z does not preciselycoincide with the laser beam. The wedge 22 has first and secondreflective surfaces 23, 24 disposed at an end thereof that intersect ata 90° angle to form an edge 25. Each of the first and second reflectivesurfaces 23, 24 are respectively disposed at a 45° angle with respect tothe central axis Z of the wedge 22 and laser device 12.

A motor 28 is coupled to the wedge 22 by a shaft 27 which enables thewedge to rotate about the central axis Z The wedge 22 may be comprisedof a thermally stable material, such as glass or quartz, and may beprovided with a reflective coating on the surfaces 23, 24. As known inthe art, the laser source 12 and wedge 22 may be further disposed withina housing 20 that provides a mechanical structure to control and permitadjustment to the relative alignment of the operative elements. Thelaser source 12 is coupled mechanically to the wedge 22 via the housing20 so that the laser device and wedge rotate in unison. The laser source12 emits a beam A directed along the central axis to the intersection 25of surfaces 23, 24 of the wedge 22. Upon uniformly striking the surfaces23, 24, the beam A divides into respective beam components B1, B2.Rotation of the wedge 22 causes the beam components B1, B2 to sweeprespective planar regions useful for planar alignment. The housing 20may further include windows or other openings that permit the beamcomponents B1, B2 to project therethrough.

A beam target 30 is disposed remotely from the wedge 22 and is orientedso that the beam components B1, B2 will impinge thereon in analternating manner as the wedge 22 rotates. The target 30 provides acentroid measurement of the beam components B1, B2, and can be providedby conventional one or two-axis lateral-effect photodiodes formed from asemiconductor material. The photodiodes produce electrical current basedupon the energy centroid of the respective laser beam components. Thephotodiodes produce an output that corresponds to a weighted sum of thecombined light energy of the beam components B1, B2 to define a singlecenter point of the combined light energy. Alternatively, the target 30could be provided by a charge coupled device (CCD) type device whichcould be configured to provide similar energy centroid data. In eitherembodiment, the centroid measurement is read out using conventionalelectronic systems. A rotational rate would be selected for the motor 28that is high enough to permit the beam components B1, B2 to appear as auniform planar surface, while low enough to not overly complicate thebeam target 30 electronics.

As illustrated in FIG. 2, the beam target 30 has spots projected thereonthat correspond to the beam components B1, B2. It should be apparentthat the beam components B1, B2 do not simultaneously impinge upon thetarget 30, but the beam components can be made to appear to impingesimultaneously by defining the image persistence of the target to begreater than the rotational rate. Ideally, the spots formed by the beamcomponents B1, B2 would overlap each other precisely if the source beamA were thermally stable. In practice, however, the instability of thesource beam A causes the spots formed by the beam components B1, B2 todiverge in a symmetrical manner along the Y-axis dimension on the target30. As a result, the impinging beam components appear on the target 30as separate spots disposed above and below the X-axis, respectively,with the distance between the spots varying with the thermal instabilityof the laser 12. The centroid measurement of the beam components B1, B2is illustrated in phantom at spot C, which remains constant andrepresents a thermally stable reference point for the planar alignmentapparatus.

FIGS. 3 and 4 illustrate a second embodiment of a laser alignmentapparatus configured to provide either linear or planar alignment. Thelaser alignment apparatus may be included within a single housingillustrated in phantom at 20. As in the first embodiment, the laseralignment apparatus comprises a collimated light source 12 provided by asemiconductor device, such as a laser diode. The light source 12provides a laser beam A that travels in a direction approximately alonga central axis Z. As noted above, the inherent instability of the laserbeam A results in the beam not precisely coinciding with the centralaxis Z.

The laser beam A passes first through a polarizing device 14 to enhancethe polarization of the laser beam A, and then through a retardationplate 16. The retardation plate 16 isolates the light source 12, and maycomprise a quarter-wavelength (λ/4) optical plate. Alternatively, theretardation plate 16 may be used to match the power of the two beamcomponents (described below), and for that purpose may comprise ahalf-wavelength (λ/2) optical plate. Further, the polarizing device 14and/or retardation plate 16 may also include anti-reflective coatingsthat prevent stray reflections of the laser beam A back toward the laserbeam source 12. Such reflections could cause undesirable oscillation ofthe laser beam source 12 that would result in temporal instability ofthe laser beam A. In this regard, the polarizing device 14 andretardation plate 16 act as an optical isolator or diode to protect thelaser beam source 12.

The laser beam A is divided into two beam components B1, B2 by abeamsplitter 46, a retroreflector 42 and a reference mirror 44. Thebeamsplitter 46 may comprise a glass plate having a coating on a lowersurface thereof that causes a portion of the laser beam A to reflecttherefrom to provide the beam component B1. The beamsplitter 46 isdisposed at a 45° angle with respect to the central axis Z to produce anapproximately 50% reflection of the beam component B1 in a directionnormal to the central axis. In the preferred embodiment, thebeamsplitter 46 is formed from optical quality glass having a highdegree of smoothness. Alternatively, a pellicle-type beamsplittercomprising a thin mylar sheet may be advantageously utilized. The beamcomponent B1 reflects collinearly off of the retroreflector 42,returning the beam component B1 back to the beamsplitter 46. Thereflected beam component B1 then passes partially through thebeamsplitter 46 in a direction normal to the central axis Z.

The portion of the laser beam A that has not reflected off of thebeamsplitter 46 passes through the beamsplitter as beam component B2.The beam component B2 follows a path along the central axis Y andreflects off of the surface of the reference mirror 44. The reflectedbeam component B2 then returns to the beamsplitter 46, and reflects offthe beamsplitter in a partially overlapping manner with the beamcomponent B1. The overlapping beam components B1, B2 (illustrated inFIG. 3 as beams B1+B2) provide a reference beam for the linear alignmentapparatus. The beamsplitter 46 may additionally be provided with ananti-reflective coating on an upper surface thereof to improve beamtransmittance therethrough and mitigate any internal reflections.Accordingly, the lower surface of the beamsplitter 46 is operative toprovide the above described reflections of the beam components B1, B2.

A retroreflector 32 disposed remotely from the laser alignment apparatusreceives the overlapping beam components B1, B2, and reflects themcollinearly back toward the laser alignment apparatus. As in the firstembodiment, a beam target 30 is oriented so that the collinearlyreflected beam components impinge thereon. As illustrated in FIG. 3, thebeam target 30 may be collocated with the laser alignment apparatushousing 20. Alternatively, the beam target 30 may be disposed remotelyfrom the laser alignment apparatus housing 20 and instead may beoriented to receive the overlapping beam components B1, B2 directly fromthe laser alignment apparatus without a retroreflector 32, or may bedisposed with the retroreflector as part of a remote sensing unit.Moreover, a plurality of beam targets 30 may be disposed at differentlocations in order to measure relative alignment therebetween.

The beam target 30 has spots projected thereon that correspond to thebeam components B1, B2. In the absence of instability of the sourcelaser beam A, the beam components B1, B2 would overlap precisely. As canbe seen in FIG. 4, however, the instability of the source beam A causesthe beam components B1, B2 to diverge symmetrically in both the X andY-axis dimensions. As a result, the impinging beam components appear onthe target 30 as separate spots disposed uniformly like mirror-imagesabout the origin of the X and Y-axes, with the distance between thespots and their relative orientation varying with the angularinstability of the laser device 12. The centroid measurement of the beamcomponents B1, B2 illustrated in phantom at spot C remains constant andrepresents a thermally stable reference point for the linear alignmentapparatus.

In order to provide a planar alignment system, the alignment apparatusfurther comprises a motor 28. The housing 20 containing the laser beamsource 12, polarization device 14, beamsplitter 46, retroreflector 42,reference mirror 44 and target 30 may be rotatable while maintainingthese elements in a proper relative orientation. The motor 28 is coupledto the housing 20 in a manner that enables the housing to rotate aboutthe central axis Z. Rotation of the housing 20 causes the overlappingbeam components B1, B2 to sweep respective planar regions, similar tothe first embodiment described above. The beam target 30 will continueto collect centroid information in the same manner described above toprovide planar alignment information. Accordingly, it should be apparentthat the same alignment apparatus can be used to provide either linearor planar alignment information.

In FIGS. 5, 6a and 6b, a third embodiment of a laser alignment apparatusconfigured to provide planar alignment is illustrated. The laseralignment apparatus is contained within a housing illustrated in phantomat 50. As in the previous two embodiments, the laser alignment apparatusincludes a collimated light source 12 provided by a semiconductordevice, such as a laser diode. The light source 12 provides a moderatelypolarized laser beam A that travels in a direction along a central axisZ, which is ideally coincident with the laser beam A. The laser beam Apasses through a polarization device 14 and a retardation plate 16 inorder to improve the polarization of the laser beam A, preclude strayreflection, and/or match beam component power, as described above.

The beamsplitter 46 of FIG. 5 operates in a similar manner to that ofFIG. 3 by dividing the laser beam A into two beam components B1, B2. Asin FIG. 3, the beamsplitter 46 has a coating on a lower surface thereofthat causes a portion of the laser beam A to reflect therefrom toprovide the beam component B1. The beamsplitter 46 is disposed at a 45°angle with respect to the central axis Z to produce a reflection of thebeam component B1 in a direction normal to the central axis Z. Unlikethe second embodiment of FIG. 3, however, the beam component B1 istransmitted outwardly from the laser alignment apparatus to provide afirst planar reference beam.

Instead of the reference mirror 44, a retroreflector 48 receives thebeam component B2 as it follows a path along the central axis Z. Theretroreflector 48 reflects the beam component B2 collinearly back to thebeamsplitter 46. The reflected beam component B2 then reflects off thebeamsplitter 46 in a direction normal to the central axis Z and directlyopposite from the direction of the beam component B1. The beam componentB2 is then transmitted outwardly from the laser alignment apparatus toprovide a second planar reference beam. The alignment apparatus furthercomprises a motor 28 coupled to the housing 50 in a manner that enablesthe housing to rotate about the central axis Z. Rotation of the housing50 causes the beam components B1, B2 to sweep respective planar regions,similar to the first embodiment described above.

A retroreflector 31 disposed remotely from the laser alignment apparatusreceives the beam component B1 and reflects it collinearly back towardsthe laser alignment apparatus. Similarly, a retroreflector 32 disposedremotely from the laser alignment apparatus receives the beam componentB2, and reflects it collinearly back towards the laser alignmentapparatus. A pair of beam targets 51, 52 are oriented so that therespective reflected beam components B1, B2 impinge thereon. As in theprevious embodiment, the beam targets 51, 52 may be co-located with thelaser alignment apparatus housing 50. Alternatively, the beam targets51, 52 may be disposed remotely from the laser alignment apparatushousing 50 and instead may be oriented to receive the respective beamcomponents B1, B2 directly from the laser alignment apparatus withoutthe respective retroreflectors 31, 32, or may be disposed with theretroreflectors 31, 32 as part of respective remote sensing units.

The beam targets 51, 52 have spots projected thereon that correspond tothe beam components B1, B2. As shown in FIGS. 6a and 6b, the impingingbeam components B1, B2 appear on the targets 51, 52 as spots disposedsymmetrically above and below the X-axis. The targets 51, 52 operatetogether as a single target to provide a centroid measurement of thebeam components B1, B2 illustrated in phantom at spot C in each target.The phantom image at spot C remains constant and represents a thermallystable planar reference point for the alignment apparatus.

It should be apparent to those skilled in the art that a single laseralignment apparatus could be constructed to operate in accordance withboth the second and third embodiments described above. Theretroreflector 42 of FIG. 3 could be disposed within the housing 20 in amanner that permits it to be manipulated into a position in front of thereference mirror 44. This way, the retroreflector 42 would then be inthe position of the retroreflector 48 of FIG. 5. The beam component B1would then project outwardly in a direction opposite from the beamcomponent B2 as in the embodiment of FIG. 5, rather than partiallyoverlapping the beam component B2 as in the embodiment of FIG. 3. Such aconstruction would yield substantial additional utility and flexibilityto a single laser alignment apparatus.

FIG. 7 illustrates a fourth embodiment of a laser alignment apparatusconfigured to provide planar alignment. As in the previous embodiments,the laser alignment apparatus includes a collimated light source 12provided by a semiconductor device, such as a laser diode. The lightsource 12 provides a laser beam A that travels roughly in a directionalong a central axis Z. As in the previous embodiments, a polarizer maybe utilized to improve the polarization of the laser beam A.

The laser beam A reflects successively off of a pair of reflectivesurfaces 62, 64 to redirect the laser beam A in a direction normal tothe central axis Z. The reflective surfaces 62, 64 may comprise separateoptical elements, or alternatively, may comprise separate surfaces of asingle optical element, such as a pentaprism or penta-mirror. Thereflective surfaces 62, 64 are further disposed within a single housing60. The housing 60 is rotatable independently of the light source 12about the central axis Z. A motor 68 coupled to the housing 60 rotatesthe housing about the central axis Z, causing the laser beam A to sweepa planar region. At the same time, the light source 12 is also coupledto a motor 28 that causes the laser beam A to rotate about the axis. Themotors 28, 68 may be controlled separately so that they rotate atdistinct rotational rates, as will be described below.

As in the previous embodiments, a beam target (not shown in FIG. 7) isoriented so that the laser beam A will impinge thereon. The beam targetmay be co-located with the laser alignment apparatus housing 60, oralternatively, the beam target may be disposed remotely from the laseralignment apparatus housing. The beam target may also be oriented toreceive the beam A from the laser alignment apparatus with or without aretroreflector as part of a remote sensing unit.

In this embodiment, the single laser beam A is utilized to provide theplanar alignment information, rather than the pair of beam components inthe previous embodiments. By rotating the housing 60 at a different ratethan the laser beam source 12, thermal fluctuations in the laser beam Awill appear as shifts in position of the laser spot impinging on thebeam target. As described previously, the beam target will average thelaser spot positions from successive passes of the laser beam A, i.e.,detect a centroid position from the successive laser spots. Therotational rate of the housing 60 may be selected to be half therotational rate of the laser beam source 12, or alternatively, aharmonic relationship between the rotational rates may be advantageouslyutilized.

In FIG. 8, a fifth embodiment of a laser alignment apparatus isillustrated which is configured to provide linear or planar alignment.As in the previous embodiments, the laser alignment apparatus includes acollimated light source 12 provided by a semiconductor device, such as alaser diode. The light source 12 provides a laser beam A, and apolarizer 14 may be utilized to polarize the laser beam A. Unlike theprevious embodiments, however, the direction of the laser beam A uponemission from the light source 12 is not normal to its ultimatedirection.

Instead, the laser beam A is directed to a polarization beamsplittercomprising a glass plate 76, a second polarizing device 78 and aretroreflector 82. The glass plate 76 has a surface coating thatreflects a portion of the laser beam A having a particular polarization.The angle in which the glass plate 76 is disposed with respect to thelaser beam source 12 is known as the Brewster angle, and is dependentupon the surface coating applied to the glass plate. Preferably, theglass plate 76 is formed from optical quality glass having a high degreeof smoothness and is disposed at a roughly 57° angle with respect to thecentral axis of the light source 12. The second polarizing device 78 maybe a conventional polarization beamsplitter.

The reflected portion of the laser beam A impinges perpendicularly ontothe surface of the second polarizing device 78. A first beam componentB1 reflects directly off of the second polarizing device 78, and asecond beam component B2 passes through the second polarizing device. Aretardation plate may additionally be utilized to isolate the lightsource 12 or to match the power of the beam components, as describedabove. The retroreflector 82 reflects the second beam component B2 backcollinearly toward the glass plate 76. Thereafter, both the first beamcomponent B1 and the second beam component B2 pass through the glassplate 76 in a collinear manner to provide a linear alignment referencebeam. As in the previous embodiments, a beam target may be utilized todetermine a centroid of the two beam components B1, B2 in order toprovide a thermally stable reference point. Also, the entire apparatusmay be rotated to provide a planar reference beam, as substantiallydescribed above.

In FIG. 9, a sixth embodiment of a laser alignment apparatus isillustrated which is configured to provide linear or planar alignment.As in the previous embodiments, the laser alignment apparatus includes acollimated light source 12 provided by a semiconductor device, such as alaser diode. As in the previous embodiment, the direction of the laserbeam A upon emission from the light source 12 is not normal to itsultimate direction.

The laser beam A is directed onto a beamsplitter cube 80, which dividesthe beam into a first beam component B1 and a second beam component B2.As known in the art, the beamsplitter cube 80 has four exposed outersurfaces 84, 88, 92, and 94. A partially reflective boundary 86 isdefined through the center of the cube 80 that effective joins theintersection of surfaces 84, 88 with the intersection of surfaces 92,94. As illustrated in FIG. 9, the laser beam A impinges onto the surface88, and is refracted inward toward the boundary 86. The refracted laserbeam A is divided at the boundary 86 into the first beam component B1that passes through the boundary and the second beam component B2 thatreflects off of the boundary. The first beam component B1 emerges fromthe cube 80 at surface 94, and the second beam component B2 emerges fromthe cube 80 at surface 92. The two beam components B1, B2 are thenprojected outwardly in a collinear manner to provide a linear alignmentreference beam. As in the previous embodiments, a beam target may beutilized to determine a centroid of the two beam components B1, B2 inorder to provide a thermally stable axial reference point. Also, theentire apparatus may be rotated to provide a planar reference beam, assubstantially described above.

In FIG. 10, a seventh embodiment of a laser alignment apparatus isillustrated which is configured to provide planar alignment. Unlike theprevious embodiments, the laser alignment apparatus includes acollimated light source 112 provided by a gas laser device containing asuitable active gaseous element, such as helium-neon gas. The lightsource 112 comprises a tubular shaped element that is capable ofemitting laser beams A1, A2 from opposite ends, respectively. The laserbeams A1, A2 are directed along a common axis of the light source 112 inopposing directions.

A motor 28 is coupled to the light source 112 in a manner perpendicularto a central axis Z of the light source. Rotation of the laser beamsource 112 by the motor 28 causes the beams A1, A2 to sweep respectiveplanar regions. As in the previous embodiments, a beam target may beutilized to determine a centroid of the two beams A1, A2 in order toprovide a planar reference point. It should be apparent that the laserbeams A1, A2 have much greater thermal stability than the laser diodesdescribed in the preceding embodiments of the invention; nevertheless,the centroid measurement would permit even greater planar alignmentaccuracy than that achievable with either laser diodes or conventionalgas laser alignment systems.

Having thus described a preferred embodiment of dual beam laser devicefor linear and planar alignment, it should be apparent to those skilledin the art that certain advantages of the within system have beenachieved. It should also be appreciated that various modifications,adaptations, and alternative embodiments thereof may be made within thescope and spirit of the present invention. For example, holographicand/or detractive optic elements may be utilized to split a source laserbeam into beam components, rather than the more conventional opticalcomponents described above. Further, beam components may be caused tosweep planar regions through an oscillatory movement in which theoptical systems are rotated through less than a full 360° of motion,then returned in an opposite direction through an equivalent range ofmotion, as opposed to a continuous rotation in a single direction. Theinvention is defined by the following claims.

What is claimed is:
 1. An apparatus for providing a reference beam foruse in an alignment system, comprising:a collimated light sourceproviding a light beam; means for dividing said light beam into a firstand second beam component, wherein said first and second beam componentsare projected outwardly from said dividing means in opposite directions;means for detecting a common centroid of said first and second beamcomponents that is independent of drift of said light beam; and meansfor rotating said dividing means about an axis to cause the first andsecond beam components to sweep respective planar regions.
 2. Anapparatus for providing a reference beam for use in an alignment system,comprising:a collimated light source providing a light beam; means fordividing said light beam into a first and second beam component, whereinsaid first and second beam components are projected outwardly from saiddividing means in opposite directions, wherein said dividing meansfurther comprises a housing and a beamsplitter disposed within saidhousing at a predetermined angle, said beamsplitter having a reflectivecoating which reflects said first beam component therefrom, said secondbeam component being transmitted through said beamsplitter; means fordetecting a common centroid of said first and second beam componentsthat is independent of drift of said light beam; and means for rotatingsaid housing about an axis to cause said first and second beamcomponents to sweep a planar region.
 3. An apparatus for providing aplanar reference beam for use in an alignment system, comprising:acollimated light source providing a light beam; a wedge having first andsecond faces that respectively reflect first and second beam componentsof said light beam in opposite directions; means for rotating said wedgeabout an axis to cause the first and second beam components to sweeprespective planar regions; and means for detecting a common centroid ofsaid first and second beam components.
 4. The apparatus of claim 3,wherein said detecting means further comprises a photodetector orientedremotely from said wedge to permit said first and second beam componentsto impinge thereon and provide said common centroid measurementtherefrom.
 5. The apparatus of claim 3, wherein said rotating meansfurther comprises a motor.
 6. An apparatus for providing a referencebeam for use in an alignment system comprising:a collimated light sourceproviding a light beam; means for dividing said light beam into firstand second beam components, said dividing means comprising:first opticalmeans disposed at a predetermined angle with respect to said light beamfor reflecting said first beam component therefrom and for transmittingsaid second beam component therethrough; second optical means forcollinearly reflecting said reflected first beam component back to saidfirst optical means, said first beam component being thereby transmittedthrough said first optical means; and third optical means for coaxiallyreflecting said second beam component back to said first optical means,said reflected second beam component being thereby reflected off of saidfirst optical means in an overlapping manner with said first beamcomponent; means for detecting a common centroid of said first andsecond beam components; and means for rotating said dividing means aboutan axis to cause said first and second beam components to sweep a planarregion.
 7. An apparatus for providing a planar reference beam for use inan alignment system, comprising:a light source providing a light beam;means for dividing said light beam into a first and second beamcomponent directed in opposite directions, said dividing meanscomprising:first optical means for reflecting said first beam componenttherefrom in a first direction and for transmitting said second beamcomponent therethrough; and second optical means for collinearlyreflecting said transmitted second beam component back to said firstoptical means, said first beam component being thereby reflectedtherefrom in a second direction opposite to said first direction of saidfirst beam component; means for rotating said dividing means about acentral axis to cause said first and second beam components to sweeprespective planar regions; and means for detecting a common centroid ofsaid first and second beam components.
 8. The apparatus of claim 7,wherein said first optical means is disposed at a predetermined anglewith respect to said central axis.
 9. The apparatus of claim 7, whereinsaid first optical means further comprises a beamsplitter having areflective coating on at least a first surface thereof.
 10. Theapparatus of claim 7, wherein said second optical means furthercomprises a retroreflector.
 11. The apparatus of claim 7, wherein saiddetecting means further comprises:at least one photodetector oriented topermit said first and second beam components to impinge thereon andprovide said common centroid measurement therefrom.
 12. The apparatus ofclaim 7, wherein said detecting means further comprises:a firstphotodetector oriented to permit said first beam component to impingethereon; a second photodetector oriented to permit said second beamcomponent to impinge thereon; and means associated with said first andsecond photodetectors for providing said common centroid measurementtherefrom.
 13. The apparatus of claim 7, wherein said light sourcefurther comprises a laser diode.
 14. The apparatus of claim 7, whereinsaid rotating means further comprises a motor.
 15. An apparatus forproviding a planar reference beam for use in an alignment system,comprising:a collimated light emitting source providing a light beam andmeans for rotating said light emitting source at a first rotationalrate; reflecting means having at least first and second surfaces forreflecting said light beam in a normal direction and means for rotatingsaid reflecting means at a second rotational rate to cause saidreflected light beam to sweep through a substantially planar region,said second rotational rate being less than said first rotational rate;and means oriented to permit said reflected light beam to impingethereon for detecting a centroid of said reflected light beam.
 16. Theapparatus of claim 15, wherein said detecting means further comprises aphotodetector oriented remotely from said reflecting means.
 17. Theapparatus of claim 15, wherein said second rotational rate is half saidfirst rotational rate.
 18. The apparatus of claim 15, wherein saidsecond rotational rate is harmonically related to said first rotationalrate.
 19. The apparatus of claim 15, wherein said reflecting meansfurther comprises a pentaprism.
 20. An apparatus for providing areference beam for an alignment system, comprising:a collimated lightsource providing a first and a second laser beam directed in oppositedirections; means for rotating said collimated light source about anaxis to cause said first and second laser beams to sweep respectiveplanar regions; and means for detecting a common centroid of said firstand second beam laser beams.
 21. The apparatus of claim 20, wherein saidcollimated light source further comprises a gas laser.